Contemporary camber induced sails, for example a boardsailing sail as shown in FIGS. 1, 2 and 3, consist of a sail 10 with a luff tube or sleeve 12 to accept a mast 14, battens 16 held in batten pockets 18 in the sail and a single or multiple camber inducers 20. A wishbone shaped boom 15 is rigidly (i.e. non-rotatably with respect to the mast 14) attached at one end to the mast 14 at a point partway up from the foot of the mast 14. The luff sleeve 12 has an opening in it to accommodate the attachment of the boom 15 to the mast 14. The sail 10 extends between the wishbone arms of the boom 15 towards its aft end where the sail 10 is pulled taut by an 20 outhaul line 19 attached between the clew of the sail and the aft end of the boom 15. The foot of the mast 14 is attached via a universal joint 7 to a sailboard 5.
A camber inducer is a coupling that connects a batten to the mast. When assembled, the sail takes the shape of an asymmetrical foil due to the force exerted on the camber inducer(s) by the batten(s) and due to the shape built into the sail by using broadseaming.
Such camber induced sails are described in U.S. Pat. Nos. 4,649,848 (Belvedere), 4,733,624 (Belvedere), 4,686,921 (Magnan), 4,708,079 (Magnan), 4,856,447 (Magnan), and 4,625,671 (Nishimura). These patents describe camber inducer devices whose primary purposes are to aerodynamically clean up the leading edge of the sail or do away with the turbulent wake found just behind the mast at the leading edge of the sail. Another important consideration of these patents is the rotational coupling of the leading ends of the battens to the mast to allow them to pivot with respect to the mast as the sail is rotated on the mast when changing tack. Although the boom 15 cannot rotate on the mast 14, the sail 10 does rotate about the mast 14 between the arms of the boom 15. Thus, on one tack the sail 10 assumes a concave foil shape as viewed in FIG. 3 but on another tack the sail 10 assumes a convex shape when viewed from the same side of the sailboard.
Still another consideration discussed in these patents is maintaining a high camber ratio with maximum draft of the sail positioned forwardly near the luff to contour the sail in an efficient aerodynamic shape irrespective of wind conditions. (See U.S. Pat. Nos. 4,625,671 and 4,686,921). Since these patents were issued, contemporary camber induced sails have evolved to the point where the sails are required to hold a rigid foil shape even during high wind conditions, i.e. when the sail is significantly loaded.
With such contemporary camber induced sails, however, when a high wind force loads the sail (FIGS. 2 and 3) the shapes of the overall foil and the leading edge are distorted and the tension is reduced. When a sail 10 is dynamically loaded, the high pressure builds on the windward side. This high pressure forces the flexible battens 16 in the sail 10 to bend and causes the center of force 22 acting on the sail 10 due to the wind, referred to as the "draft" of the sail, to move aft. Also, the force on the skin of the sail 10 locks the battens 16 in their pockets 18 in the aft section of the sail 10 and the result is that the battens are pulled away from the front of the sail. This does two things. It detensions the leading edge of the sail 10, i.e. slackens the sail in the fore and aft direction, and derotates the leading edge, i.e. flattens the degree of curvature, if not completely inverting it. This can be seen in both FIGS. 2 and 3. In FIG. 3 the effect is noticeable in the slight "S" shape imparted to the battens just aft of the luff sleeve 12.
Both of these effects reduce the aerodynamic efficiency of the sail. The detensioning also makes the sail 10 unstable and vulnerable to complete inversion in the event that the angle of attack to the wind is decreased. The altered foil shape develops less lift, has more drag and draft instability, and is very difficult to keep balanced and use in an effective sailing position.
One obvious solution to these problems is to use stiffer battens; however, this introduces additional problems. A boardsailing sail must be able to freely rotate about the mast during tacking. While the stiffer battens have proven to keep the draft from shifting, they do not overcome the problems of detensioning and derotation during loading. Furthermore, stiffer battens make rotation of the sail 10 about the mast during tacking more difficult.